Now consider a situation where you need to create collection <code>mypython</code> that depends on <code>python26</code> collection that defines macro <code>%{__python2}</code> in a way shown above. The macro will evaluate to <code>/opt/<provider>/mycollection/root/usr/bin/python2</code>, but the <code>python2</code> binary is not in your collection, but in <code>python26</code> - this means nothing can be built in depending collection now. When you need to follow this use-case, here is what you need to do:

Now consider a situation where you need to create collection <code>mypython</code> that depends on <code>python26</code> collection that defines macro <code>%{__python2}</code> in a way shown above. The macro will evaluate to <code>/opt/<provider>/mycollection/root/usr/bin/python2</code>, but the <code>python2</code> binary is not in your collection, but in <code>python26</code> - this means nothing can be built in depending collection now. When you need to follow this use-case, here is what you need to do:

Naming the SCL

Every SCL must have a name which does not match name of any other package. This is implied by the fact that the SCL metapackage's name is the same as name of the whole SCL. It is therefore not a good idea to name an SCL "ruby", when there is a "ruby" package - the name has to be altered. Packager should include something specific to the SCL (usually a specific version of included library) into it's name. Therefore "ruby193" is a good name for a collection that contains Ruby 1.9.3, etc.

SCL Metapackage

Every SCL must have a so called metapackage. This metapackage must have a runtime and build subpackages (the build subpackage is needed for Koji builds, but may be left out for local builds).

MANPATH=%{_mandir}:\${MANPATH} to be able to diplay manpages present in the SCL

PKG_CONFIG_PATH=%{_libdir}/pkgconfig\${PKG_CONFIG_PATH:+:\${PKG_CONFIG_PATH}} or PKG_CONFIG_PATH=%{_datadir}/pkgconfig\${PKG_CONFIG_PATH:+:\${PKG_CONFIG_PATH}} to enable using pkg-config files

XDG_DATA_PATH=%{_datadir}\${XDG_DATA_PATH:+:\${XDG_DATA_PATH}} to use systemtap tapsets from the SCL

If you use any of the above, that contains %{_libdir}, the metapackage must be arch specific, otherwise it may be noarch.

Among other things, the %scl_install macro creates a macro file macros.%{scl}-config (that ends up in the %{scl}-build package). This file is always located at %{buildroot}%{_root_sysconfdir}/rpm/macros.%{scl}-config. If you need to add some more macros specific for this SCL, add them into this file.

Unlike in specfiles of normal SCL packages (see below), the metapackage doesn't need to conditionalize SCL specific macros, as it can only be used as a part of SCL (for example, instead of Requires: %{?scl_prefix}foo, use just Requires: %{scl_prefix}foo - notice the missing questionmark, that makes the first macro conditional).

When user runs yum install ruby193, he expects that the whole SCL with all dependencies gets installed. Because of that, it should have Requires on all packages of that SCL, that are needed for the SCL to fulfil its purpose. E.g. yum install ruby193 definitely has to install Ruby 1.9.3, but it needn't install collection packages that were used to build the interpreter (and that are not needed at Ruby runtime).

Converting Packages for SCL

This section sums up the steps needed to take in order to convert a normal specfile to SCL specfile step by step. The resulting package should be buildable even without SCL - that means both without %{scl}-build in the buildroot and with it. Advantage of this approach is, that both non-SCL packages like rubygem-foo and SCL packages %{scl_prefix}rubygem_foo (depending on the SCL) can be built from the same SRPM, depending on the buildroot.

Automationspec2scl was written to automate the task of converting specfiles to scl-enabled specfiles as much as possible. Install it by yum install spec2scl or use upstream version from https://bitbucket.org/bkabrda/spec2scl.

Converting Tags and Macro Definitions

Every SCL specfile must have %scl_package macro specified (like any other macro, it must be conditionalized). This macro does this:

Rewrites the standard path macros by prefixing them with %{_scl_prefix}/%{scl}/root/. E.g. %{_datadir} changes (depending on value of %_scl_prefix) from /usr/share to e.g. /opt/fedora/myscl/root/usr/share.

Introduces some SCL specific macros (like %pkg_name, %scl_prefix or %_root_* set of macros, that contain values of the original RPM macros (e.g. %_root_datadir contains /usr/share).

One of the important macros is %pkg_name, which represents the original package name - the %name macro stands for the name with SCL prefix during SCL build. Therefore it is a good practice to also define %pkg_name macro for non-SCL builds, to be able to use it consistently throughout the whole specfile.

So here is what the first two lines should look like:

%{?scl:%scl_package foo}
%{!?scl:%global pkg_name %{name}}

Usual steps to adapt tag definitions for SCL builds are these:

Name must be modified like this:

-Name: foo
+Name: %{?scl_prefix}foo

Requires and BuildRequires have to be considered carefully. These depend on what you are building/linking with and it is your decision as a packager. The only rule here is, that if building/linking with other SCL packages, their names must be also prefixed with conditionalized macro %{?scl_prefix} like this:

-Requires: bar
+Requires: %{?scl_prefix}bar

For example, since ifconfig is a binary, we're going to use its system version, which means not using %scl_prefix with it. rubygem-foo, on the other hand, has to be in the path of the Ruby interpreter which will be part of this SCL, therefore it needs to be used with %scl_prefix. When depending on system packages, you should be very general in your requirements (avoid using versioned Requires on specific versions) and if you need a package that might be updated, you have to either add it to your SCL or be willing to rebuild the SCL when the system package updates.

Obsoletes, Conflicts and BuildConflicts must always be prefixed with %{?scl_prefix}. This is extremely important, as the SCLs are often used for deploying new packages on older systems (that may contain old packages, now obsoleted by the new ones), but they shouldn't Obsolete or Conflict with the non-SCL RPMs installed on the system (that's the idea of SCL :) ). For example:

-Obsoletes: foobar < 1.0
+Obsoletes: %{?scl_prefix}foobar < 1.0

Provides tag must always be prefixed with %{?scl_prefix}. For example:

-Provides: foo(bar)
+Provides: %{?scl_prefix}foo(bar)

All the other tag definitions should be unchanged, unless they contain %{name} macro, which may need to be substituted for %pkg_name (for example in SourceN tag, where it may be a part of URL).

There are some additional tags you should add. The package must require %{scl}-runtime, unless it depends on another package that requires %{scl}-runtime.

Here is a rule of thumb: If packaging SCL with a language interpreter, like Ruby or Python, typically all other packages in the SCL depend on the interpreter. Therefore it is sufficient when only the interpreter runtime package requires %{scl}-runtime. Generally, every SCL package that can be installed on its own (without other packages from this SCL) should require the %{scl}-runtime package.

The line to add is:

%{?scl:Requires %{scl}-runtime}

Subpackages

The same rules as for normal tags apply for subpackages tags. The only thing needs to be changed:

If (and only if) a package define its name with -n, the name must be prefixed with %{?scl_prefix} like this:

-%package -n foo
+%package -n %{?scl_prefix}foo

This applies not only to %package macro, but also for %description and %files.

Inter-SCL Dependencies

Available from scl-utils version 20120613.

There are situations where packages from an SCL have to depend on packages from another SCL. While using simple [Build]Requires: ruby193-rubygem-rails is possible, it is not the wisest thing to do, since it hardcodes the information about how %{?scl_prefix} gets expanded. If the expansion of this macro was changed (for example from ruby193- to scl-ruby-193, all the packages using such dependencies would need to be manually altered and rebuilt.

The scl-utils package therefore provides two ways of inter-SCL dependencies using macros. Note that both of them are in fact functions, so they need to be invoked without the curly braces to work properly:

%scl_require foo macro is used to depend on the whole SCL. This is used if you want to use a functionality of an SCL, no matter what packages that SCL will install along the way. For example, the ruby193 SCL contains the whole Ruby on Rails stack in version 3.2.3. If packaging an SCL that requires the whole Rails stack, you can simply use %{?scl:Requires: %scl_require ruby193}.

%scl_require_package foo pkg provides the ability to depend on a specific package from another SCL. For example, if your package needs rubygem-minitest package from ruby193 SCL, you can simply use %{?scl:BuildRequires: %scl_require_package ruby193 rubygem-minitest}. Installing the whole Rails stack in this case would be useless (and since rubygem-minitest is only a build dependency, it's not drawn in by default).

Note that both of these macros abstract from the way that %{?scl_prefix} expands. Therefore, if any changes were made, a simple rebuild with new versions of SCL utils would be sufficient.

When specifying inter-SCL dependencies, these macros must be used.

Inside RPM Scripts

It is not possible to describe the general process of rewriting %prep, %build, %install, %check and the %pre* and %post* scripts. There are however some general rules:

Substitute all occurencies of %name for %pkg_name. Most importantly, the %setup macro will need the -n argument for SCL builds (thanks to %pkg_name, we can use the same for non-SCL builds). Due to this, the %setup macro must always be used:

-%setup
+%setup -n %{pkg_name}-%{version}

If using a %_root_* macro to point to the original filesystem, you must conditionalize it, so that the package can be rebuilt for non-SCL use:

If building SCL packages that depend on other SCL packages, you might need the scl enable functionality to link properly/run proper binaries, etc. It is not generally possible to say where this will be needed, but an example might be compiling against an SCL library or running an interpreted script with the interpreter in SCL:

All hardcoded paths (such as /usr/share) must be replaced with proper macros (%{_datadir} in this case).

Files

The files section is usually OK as it is. The only adjustments needed are for the names of subpackages (see Subpackages) and possible of some path macros with their %_root_* alternatives (this is determined by what you do in the scripts, see Inside RPM Scripts.

Dealing With Automatic Provides/Requires and Filtering

RPM has some automatic Provides/Requires searching capabilities as well as filtering capabilities. For example all Python libraries have an automatically added Requires: python(abi) = (version) (and in SCL, the proper way to have this is Requires: %{?scl_prefix}python(abi) = (version)). The scripts that search for these dependencies must sometimes be rewritten for SCL, as the original RPM ones are not extensible enough (and in some cases, filtering is not usable). This is the example of rewriting Python provides and requires (the following lines can be placed into the macros.%{scl}-config):

Macros and Depending Collections

Now consider a situation where you need to create collection mypython that depends on python26 collection that defines macro %{__python2} in a way shown above. The macro will evaluate to /opt/<provider>/mycollection/root/usr/bin/python2, but the python2 binary is not in your collection, but in python26 - this means nothing can be built in depending collection now. When you need to follow this use-case, here is what you need to do:

In macros.python.python26 (part of python26-python-devel), define

%__python26_python2 /opt/<provider>/python26/root/usr/bin/python2

And in macros file in python26-build and any depending collection's %{scl}-build use:

%scl_package_override() {%global __python2 %__python26_python2}

This will redefine the %{__python2} macro only if such %{scl}-build package is present (which usually means that you want to build for the collection and don't mind breaking the macro for local non-SCL builds).

Macro %scl_package_overrideThis macro function is called at the end of %scl_package function. It is not defined in scl-utils-build, it is rather left to be defined by SCL packagers in %{scl}-build, so that they can override arbitrary macros only during collection package build. Due to the fact that %scl_package is always called from specfile, macros (re)defined by this function effectively override any macro defined in RPM macro files.

Dealing With Shebangs

Shebangs have two important aspects: they are processed by the automatic dependency processor and they point to a certain (possibly non-SCL location).

As one of its functions, the automatic dependency processor goes through the shebangs. And adds dependencies according to the binaries they point to. From the SCL point of view, there two types of shebangs:

/usr/bin/env foo

'auto-dependency generation' point of view: The resulting package will depend on /usr/bin/env, which is not a problem.

'pointing to non-SCL location' point of view: If the $PATH is redefined properly in the enable scriptlet, the foo binary is found in the SCL hierarchy, so not a problem either.

But you generally should avoid this. Packages that belong into collection should make sure that they invoke collection binaries, so they should hardcore full path to them in shebangs.

/usr/bin/foo

'auto-dependency generation' point of view: The resulting package will depend on /usr/bin/foo (part of non-SCL package), but we may have wanted to depend on %{?_scl_root}/usr/bin/foo when building for SCL.

'pointing to non-SCL location' point of view: The $PATH redefinition has no effect, the /usr/bin/foo binary is still used (which is probably not what was intended).

If you have some shebangs like this in the specfile and the should point to SCL locations when built for SCLs, you can use similar command to adapt them:

Building Packages

The converted packages should be buildable both in SCL and non-SCL buildroots.

Before building in mock, you will want to create SRPMs. The process here is standard - if the SCL macroes are properly conditionalized, everything will work even without the SCL metapackage and scl-utils-build installed locally. However, for the build of the metapackage itself, you will need scl-utils-build, otherwise rpmbuild will be unable to parse the macros inside it.

For testing the SCL builds in mock (you will get the same environment in BREW), you have to create a standard config file with few adjustments (let's say that you are building SCL named ruby193):